This study investigated how the lexical and syntactic features of two Hungarian advanced English as a Foreign Language (EFL) learners’ writing evolved over a period of four months from a dynamic systems theory (DST) perspective. The participants provided data in the form of written essays which were analysed computationally by software packages (CohMetrix 2.0, Coh-Metrix 3.0 and Synlex L2 Complexity Analyzer). It was found that both lexical and syntactic indices showed interindividual and intraindividual variability. The log frequency for content words index showed a gradual decline which suggests that both participants started to use less frequent lexical items in their writing. When measure of textual lexical diversity (MTLD) was plotted against mean length of Tunit (MLTU) and MTLD against dependent clause per T-unit (DC/T), it was found that both participants concentrated on lexical complexity rather than on syntactic complexity which
was also confirmed by the interview data. The largest rate change occurred for coordinate phrases per T-unit (CP/T) for both participants.